Many researchers have suggested that implementing a “natural flow regime” will restore riparian landscapes and thwart invasion of non-native species in regulated rivers.  However, the phenology of Tamarix, a prolific invader of riparian areas, seems well-suited towards hydrographs that are dominated by spring floods, and observations of high rates of Tamarix invasion on many unregulated tributaries and river reaches (e.g. Colorado River through Cataract Canyon) do not support the natural flow regime paradigm.  Our study was designed to explore the possibility of using river regulation to prevent further Tamarix establishment and favor establishment of native species in the Grand Canyon National Park (GCNP).  This involves learning more about the life history and demography of Tamarix and native shrubs and how these traits interact with the dynamic riparian landscape.  Dendroecological techniques in combination with analyses of historical aerial photography allow us to precisely correlate specific Tamarix establishment events with temporal variation in flow regime and climate, as well as spatially heterogeneous patterns of geomorphic surface types and particle size distribution.  Preliminary results indicate that Tamarix establishment is facilitated by alternating years of high and then low flow, although a background rate of establishment is maintained by a recent, managed flow regime of low daily and seasonal fluctuations.  Random surveys for seedlings indicate that seasonal increases in water levels may currently induce mortality in Tamarix seedlings.  Through statistical modeling of establishment patterns on regulated and unregulated reaches, we will address the appropriateness of using landscape-level flow manipulations to manage the Tamarix invasion.